Add `--install` option to `sv create` and `sv add`

[Rich-Harris]

If it were possible to specify an --install option, it would mean that sv create and sv add could be fully non-interactive provided all other options were specified:

# use the default package manager
npx sv create --install

# use the specified package manager
npx sv create --install=pnpm

[hyunbinseo]

Is there a reason to use npx if the desired package installer is pnpm?

npx sv create --install=pnpm # why do this
pnpm dlx sv create # when this can be done